In the generation of electricity steam generators are typically used to power turbines that are connected to electrical generators. In some installations it is necessary or desirable to provide some means of isolating a steam generator from the turbines and other portions of the steam system. In nuclear-powered generating units it is necessary to be able to substantially instantaneously isolate the steam generator from the remainder of the system for safety reasons. Isolation valves normally provided for steam generators are of the gate valve type, however, they are not provided with actuators which will close the valves quickly. In nuclear-powered electric generating stations much emphasis is placed on safety and therefore a dual control system is required for operating many elements. Additionally the operation elements in these generating stations must be constructed so they will close to shut down the system in the event of a failure or an emergency. In the art many actuators and control systems are known for opening and closing of valves; however they are not particularly adapted for opening and closing large high pressure gate valves such as those used with steam generating power plants. The prior art valve actuators and control systems are constructed to close the large valves relatively slowly. Additionally these control systems do not have the redundancy which is necessary for operating in a nuclear power generation system environment. Valve control systems are known in the art which have a piston and cylinder apparatus connected to the valve closure member wherein air or some other gaseous fluid is on one side of the piston and hydraulic fluid is on the opposite side of the piston. These control systems are used for controlling the position of the valve closure member so it can be used in a partially open position to regulate fluid flow through a line. This is illustrated in U.S. Pat. No. 2,918,085.
Gate valves employed heretofore in such power plants have not been of the expanding gate valve type in which a gate and segment carried thereby are cammed outwardly at the end of a stroke into tight sealing relation with the seats in a metal-to-metal sealing contact with the gate and segment. Gate valve seats for high temperature service above around 450.degree. F. require a metal-to-metal sealing surface as Teflon or rubber seals will be destroyed by high temperatures such as encountered in steam service which may be as high as around 850.degree. F. Wedge type gates and gate having springs to effect a metal-to-metal seal by urging a gate member outwardly have been employed heretofore in such high temperature service but such gates have not utilized a gate and segment in which the segment is first stopped adjacent the end of a stroke and the gate is then cammed outwardly into fluid-tight metal-to-metal contact with the seats for the remainder of the gate travel.